1. Field of the Invention
The present invention relates to a rotary drive apparatus for electronically controlling a rotary angle of a rotary body. More particularly, it relates to a rotary drive apparatus which can effectively operate, even inside a bar of magnetic material, such as a shock absorber for a vehicle.
2. Description of the Related Art
In a conventional electronically controlled suspension unit for a vehicle, the damping effect derived from oil contained in a cylinder of the suspension unit can be controlled by changing the flow area of orifices through which the oil flows. This control is attained by a motor rotating a rotary rod within a piston in the cylinder, thus moving a gear which controls the opening and closing of a rotary valve to selectively communicate or not communicate with the orifice. However, torque generated by the motor is transfered to the far end of the piston through the rotary rod, and therefore, the sealing and controllability of the system become difficult.
To alleviate this problem, an actuator is incorporated within the piston rod. Since the piston rod is a part of the suspension unit, preferably a rotary-type actuator without a spring is incorporated, to lessen the effects of vibration thereon.
In the above case, the actuator incorporated within the piston rod is usually a conventional rotary solenoid, which includes coils, an iron yoke, yoke magnetic poles, and permanent magnets--all of which are fitted into holes made inside the piston rod. In this construction, the coil is energized to generate magnetic poles at the yoke magnetic poles. However, in this conventional construction, the problem arises wherein, if the above structure is inserted into the piston rod, the magnetic poles are short-circuited because the piston rod is made of a magnetic material, and thus the magnetic poles cannot produce an effective magnetic field for the permanent magnets to rotate same.
To overcome this problem, it has been proposed to divide the yoke into separate yokes, and to wind the coils around each of the yokes separately. However, this produces the problem of allowing more space for the coil windings, and of the smaller size of yokes made necessary by space limitations.
The above conventional structures will be explained later in more detail, with reference to the attached drawings.